1. Field of the Invention
The invention relates to a control system and control method for an internal combustion engine, which are able to estimate the concentration of sulfur in fuel of the internal combustion engine with the use of a limiting current gas sensor.
2. Description of Related Art
There is known a limiting current gas sensor as a gas sensor that is arranged in an exhaust passage of an engine. The limiting current gas sensor includes a pumping cell. The pumping cell includes a solid electrolyte and a pair of electrodes. The solid electrolyte layer has ion conductivity. The pair of electrodes are respectively fixed to both faces of the solid electrolyte layer. One of the pair of electrodes is exposed to a test gas (that is, exhaust gas) that is introduced through a diffusion-controlling layer, and the other one of the pair of electrodes is exposed to the atmosphere.
In the limiting current gas sensor, when a voltage that is applied between the pair of electrodes (hereinafter, also referred to as inter-electrode applied voltage) is set to a predetermined voltage within a limiting current region, a current according to the concentration of specific components (for example, oxygen, nitrogen oxides (NOx), and the like) in the test gas flows between the pair of electrodes. This current is also termed electrode current. The limiting current gas sensor outputs a physical quantity (for example, a voltage) according to the electrode current.
For example, a limiting current gas sensor described in Japanese Patent No. 4664882 includes electrodes containing a material, such as rhodium (Rh) and platinum (Pt), having a high NOx reduction property. Such a limiting current gas sensor decomposes NOx in exhaust gas into N2 and O2 by applying a predetermined voltage to the electrodes, and generates an electrode current according to the amount of O2. Thus, the concentration of NOx is acquired by measuring the electrode current of the limiting current gas sensor.
Fuel (such as light oil and gasoline) of an internal combustion engine mounted on a vehicle may contain a small amount of sulfur (S). When the concentration of sulfur in fuel (hereinafter, also simply referred to as sulfur concentration) is high, there is a possibility that the engine degrades or white smoke is produced. In addition, there is a possibility that sulfur poisoning of an exhaust gas control device interposed in an exhaust passage of the engine advances.
Therefore, it is desirable that the concentration of sulfur in fuel be detected and then the detected sulfur concentration be reflected in control over the engine, an alarm related to a failure of the engine be issued or the detected sulfur concentration be put to use in improvement of an on board diagnosis (OBD) on the exhaust gas control device.
Incidentally, when fuel of the engine contains sulfur (S), sulfur oxides (SOx) are contained in exhaust gas that is emitted from a combustion chamber. The concentration of SOx in exhaust gas (hereinafter, referred to as exhaust gas SOx concentration or also simply referred to as SOx concentration) increases as the concentration of sulfur in fuel increases. Thus, it is conceivable that, if the exhaust gas SOx concentration is measured, it is possible to estimate the concentration of sulfur in fuel on the basis of the measured exhaust gas SOx concentration.